In recent years, there is discussed “Heterogeneous Network (HetNet)” as one method for increasing a network capacity for increasing traffics.
In “HetNet”, a plurality of picocells are located in a conventional macrocell so that traffics such as hot spot are absorbed in the picocells while a coverage is ensured in the macrocell, thereby increasing the capacity of the entire network.
In this way, when a large number of picocells are located, many picocells using the same PCI (Physical Cell Identity) are likely to be present in an area (or a neighboring area) of one macrocell (radio base station eNB). Therefore, there is a problem that the macrocell (radio base station eNB) cannot uniquely designate a picocell due to the neighboring picocells with only the PCI.
Handover from macrocell to picocell (or femtocell), or “issue on PCI confusion” for “inbound HO” is defined in its solution in the LTE (Long Term Evolution) Release-9 system.
With the solution, CGI (Cell Global Identity) of a neighboring picocell (femtocell) with better reception quality is reported from a mobile station UE, thereby uniquely determining a handover-destination cell in the macrocell (radio base station eNB).
FIG. 6 illustrates a reconnection procedure in the LTE Release 8/9/10 systems.
As illustrated in FIG. 6, in the LTE Release 8/9/10 systems, in step S3001, when detecting RLF (Radio Link Failure) in cell #1 under a radio base station eNB#1, a mobile station UE starts a reconnection procedure for a radio base station eNB#2 managing cell #2 selected in a cell selection processing.
At this time, if the reconnection destination radio base station eNB#2 is managing “UE Context” for the mobile station UE, the reconnection procedure is successfully performed.
In step S3005, the radio base station eNB#2 compares “source PCI”, “source C-RNTI (Cell-Radio Network Temporary Identity” or “short MAC-I” transferred from the radio base station eNB#1 in a “HO preparation processing (step S3002)” with “source PCI”, “source C-RNTI” or “Short MAC-I” contained in “RRCConnectionReestablishmentRequest” transmitted by the mobile station UE, thereby performing a “matching processing (mobile station UE designating processing)” and a “verification processing” on “UE Context.”
The radio base station eNB#1 transfers a security parameter (such as algorithm, KeNB* or NCC) to the radio base station eNB#2 in the “HO preparation processing (step S3002).”
The security parameter is required to keep security between the mobile station UE and the radio base station eNB#2 after completion of the reconnection procedure.
When the mobile station UE is designated and verified, in step S3006, the radio base station eNB#2 transmits “RRCConnectionReestablishment” to the mobile station UE.
Then, communication after “RRCConnectionReestablishmentComplete” in step S3007 is made with a security processing (“ciphering” or “integrity protection”).
“SRB1 “Signaling Radio Beater 1)” is set by “RRCConnectionReestablishment” in step S3006, and then “SRB2” or “DRB (Dedicated Radio Bearer)” is reconfigured by “RRCReconfiguration” in step S3011 and the communication is continued.
The routing in the network is switched by “S1 path switch request” in step S3008 and “S1 path switch response” in step S3009.
Herein, if the “matching processing” and the “verification processing” in step S3005 fail, the radio base station eNB#2 transmits “RRCConnectionReestablishmentReject”, instead of “RRCConnectionReestablishment” to the mobile station UE in step S3006.
When receiving “RRCConnectionReestablishmentReject”, the mobile station UE transits to an “Idle state.”